Process for preserving wood



i atenteci Mar. 31, 1942' UNlTED STATES PATENT OFFICE 2,277,660 PROCESS FOR PRESERVING WOOD Ernest R. Boller, Clevelandlieights, Ohio, assignor to E. I. du Pont de Nemours.& Company, Wilmington, Del., a corporation ol. Delaware No Drawing. Application February 21, 1939,

Serial No. 257,587

(Cl. 213.7) I

6 Claims.

This invention relates to the preservation of good and similar fibrous cellulosic materials by impregnation with aqueous solutions containing zinc ions and anions of an acid of arsenic and describes processes for fixing the zinc and arsenic in the fibers of the material which are characterized in that the impregnated material, while containing water in excess of the fiber saturation point, is heated under conditions such that a substantial portion of the impregnated matevrial is subjected to a temperature of at least about 220 F. for at least about one-half hour.

The impregnation ofwood and similar fibrous cellulosic materials withaqueous solutions containing zinc ions and anions of an acid of arsenic =54; inch square and 3 feet long, were treated with a preservative solution containing 0.95% zinc oxide and 1.43% arsenious acid (expressed as are well known in the art. Thus United States Letters Patent to Leo P. Curtin 1,659,135 granted February 14, 1928, 1,900,162 granted March 7, 1933, and 1,984,254 granted December 11, 1934, disclose impregnating solutions adapted to deposit in the fibers of the material treated a relatively insoluble compound of zinc and arsenious acid. A similar process using arsenic acid is diswhile containing water in excess of the fiber saturation point, is then heated to a temperature of at least 220 F. for a suitable period, .say one-half hour to one hour or more, depending upon the size of the material treated. Saturated steam at superatmospheric pressure is preferably used as a heating medium not only because it is readily available in practically all wood preserving installations but also because it prevents undue drying duringthe heating and permits suitably high temperatures without injury to the wood.

The following example illustrates the increased preservative value obtainable by the practice of my invention. Southern yellow pine specimens,

A5203). The net retention of zinc arsenite was 0.25 lb. per cubic foot of wood. A number of the specimens were then steamed for a period of one hour at 260 F. The remaining specimens were not steamed. The specimens were air-dried at normal temperatures, then set out in the ground at test plots under conditions of severe exposure representing accelerated testconditions. Comparison of the amount of decay after twelve and eighteen months exposure is made in the following table:

Performance rating in per cent specimens in category after- Condition of specimens after exposure 12 mos. exposure 18 mos. exposure Steamed I Unsteamed Steamed Unsteamed Good 75 25 Doubtful. 21 8 l7 Shallow decay. 4 59 42 67 Deep decay 33 12 33 Failed 4 It is clearly evident from these data that the steam treatment materially improved the permanence of the preservative treatments; in other words, that the zinc and arsenic in the test specimens (those steamed) was more resistant to leaching on weathering in contact with moist earth than the control specimens (those not steamed).

While the above example sets forth particular.

conditions it will be understood that it is merely illustrative and that numerous Variations may be made therein without departing from the-spirit of the invention as long as the impregnated material, while containing water in excess of the fiber saturation'point, is heated under conditions such that a substantial portion of the impregnated material is maintained at a temperature of at least about 220 F. for at least about one-half hour.

No general rule can be given as to the specific time and temperature required for fixation. These two factors are not only interrelated but are dependent upon the initial temperature and besides on the kindof woodbeing treated. It

is obvious that a'longer time will be required to v heat a largepiece of timber than a small piece.

In the same manner the higher the initialtemperature the shorter the time that will be required to raise the impregnated material to the desired temperature. It isalso much easier, for instance, to flx the preservative in Southern yellow pine than in'hardwood such as oak. In any case, however, the impregnated materialshould be held at a temperature of at least about 220 F. for at least about one-half hour.

While those skilled in the art with a little experience will be able to select suitable temperatures, depending upon the size of the impregnated material and the kind of wood impregnated, it is generally desirable to work at the highest temperature possible without injury to wood since higher temperature will permit a shorter duration of the treatment. Yellow pine, for example, may be subjected to a steam pressure of about twenty pounds per square'in'ch gage (260 F.) for a considerable time without any deleterious effect. those corresponding to about 25 pounds per square inch steam pressure (266 F.), may be used with little if any deleterious effect. For most purposes I have found it desirable to hold the impregnated material at a temperature between about 240 F. and 260 F. for from about one-half hour to one hour.- In the case of any sizable piece of timber it will be sumcient, for

Evenhigher temperatures, such-as example, to steam'the wood for a time necessary to raise thetemperature at the center of the piece from about 240 F. to about 260 F. It will be understood, however, in view of the complexity of the factors involved that the treating conditions may be varied widely and that those given represent the preferred procedure with yellow pine.

The water content of the wood at the time of heat treatment is of particular importance but is not exceptionally critical as long as water is present in excess of the fiber saturation point.

This will ordinarily vary, depending upon the kind of wood, from about 28 to 35 per cent moislution containing zinc ions and anions of an M1 of arsenic the method of fixing the zinc and arsenic in the fibers of the material which comprises heating the impregnated material while containing water in excess of the fiber saturation point under conditions such that a substantial portion of the impregnated material is heated to a temperature of at least about 220 F, for at least aboutone-half hour said heating being effected with'the material out of contact with impregnating solution containing zinc or arsenic.

2. In a process for the preservation oi wood and similar fibrous cellulo'sic materials in.which the fibers are impregnated with an aqueous solution containing zinc ions and anions of an acid of arsenic the method of fixing thezinc and arsenic in the fibers of the material which comprises heating the impregnated material while containing water in excess of the fiber saturation point under conditions such that a substantial portion of the impregnated material is maintained at a temperature between about 240 to 260 F. for at least about one-half hour said heating being effected with the material out of contact with impregnating solution containing zinc or arsenic.

3. In a process for the preservation of wood andsimilar fibrous cellulosic materials in which ture figured on the basis of the dry weight of I the wood. As a practical matter the moisture content of the wood will be satisfied if seasoned wood is impregnated with preservatives and.

thereafter .heat treated as described.

While the most satisfactory results are obtained by the use of steam as the heating medium, my invention is not limited in this respect since any heating media suitable for heating wet wood to the proper temperature and for the proper time is suitable. As a practical matter, however, heating media such as dry hot air or similar gases, which at the specified temperatures would cause rapid loss of water from the outside portions of the wood, should be avoided since the resulting shrinkage in connection with the swollen condition of the inside portion of the wood might tend to cause checking and warping. Ordinarily, however, the treatment need not be continued'long enough to give riseto these conditions but, should it be desirable to continue the heating for a longer period, excessive loss of water from the outside portions of the wood may be avoided by saturating such a heating media with water vapor or by effecting the heating in a closed retort which would restrict excessive evaporation of water. In some cases it may be advantageous to carry out the heating in an oil bath. Such a treatment offers advantages in that by the application of the proper pressure the woodmay also be impregnated with a suitable preservative oil.

I claim:

1. In a process for the preservation of wood and similar fibrouscellulosic materials in which the fibers are impregnated with an aqueous sothe fibers are impregnated with an aqueous solution containing zinc ions, and anions of an acid of arsenic the method of fixing the zinc and arsenic in the fibers of the material which comprises subjecting the impregnated material to heat treatment with saturated steam at superatmospheric pressureat a temperature between about 240 F. and about 260 F. for at least about one-half hour.

4. In a process for the preservation of wood and similar fibrous cellulosic material the steps of impregnating the fibers of said material by treatment with an aqueous solution containing zinc ions and anions of an acid of arsenic, withdrawing said solution and thereafter heating the impregnated material while containing water in excess of the fiber saturation oint under conditions such that a substantial portion of the impregnated material is heated to a temperature of at least about 220 F. for at least about onehalf hour. a

5. In a process for the preservation of wood and similar fibrous cellulosic material the steps of impregnating the fibers of said material by treatment with an aqueous solution containing zinc ions and anions of an acid of arsenic, withdrawing said solution and thereafter heating the impregnated material while containing water in excess of the fiber saturation point under conditions such that a substantial ortion of the impregnated material is maintained at a tempera- .ture between about 240 to 260 F. for at least about one-half hour.

. -6. In a process for the preservation of wood and similar fibrous cellulosic material the steps of impregnating the'fibers of said material by treatment with an aqueous solution containing zinc ions and anions of an acid of arsenic, withdrawing said solution and subjecting the impregnated material toheat treatment with saturated steam at superatmospheric pressure at a temperature between about 240 F. and about 260 F. for at least about one-half hour.

ERNEST R. BOLLER. 

